In densely spaced WDM transmission systems, two main distortions affect the signal: inter-symbol interference induced by tight filtering and crosstalk between neighboring channels.
Previously, the value of the channel spacing or grid spacing in conventional WDM transmission systems is usually chosen to be slightly higher than the value of the bitrate of the channel signal included within the individual channels, e.g. a channel spacing of 50 GHz is usually used for 43 Gb/s channel signals or for a 112 Gb/s polmux-QPSK channel signal (i.e. a channel signal using a quaternary phase-shift keying modulation method in which each modulation symbol is present in two different polarization states).
For future transmission systems, an even closer channel spacing is envisioned. Optical WDM transmission systems which transmit four duobinary modulated channel signals at a bitrate of 28 Gb/s on a 50 GHz grid are already available. Optical duobinary modulation may be used as disclosed in U.S. Pat. No. 5,892,858, which is spectrally more efficient and also more cost efficient than above polmux systems as direct detection at the receiver side can be used. Furthermore, electrical pre-distortion may be utilized to pre-compensate transmission distortions (see e.g. No. U.S. Pat. No. 7,382,984 B2 for dispersion pre-compensation). Pre-distortion methods have the advantage that complexity is lower than in case of methods using post-compensation of distortions (at the receiver). In the light of crosstalk reduction between WDM channels, also orthogonal polarization launch has been proposed for reduction of nonlinear fiber effects like four photon mixing (FPM) (see e.g. U.S. Pat. No. 6,363,181 B1). However, this application of orthogonally polarized neighboring optical channel signals within a WDM signal is used for reduction of nonlinear crosstalk, which occurs on multi-span transmission systems using low dispersion fibers.